1. Field of the Invention
The present invention concerns an x-ray tube of the type having a thermionic emitter.
2. Description of the Prior Art
In the event of failure of the electron emitter of an x-ray tube, the x-ray tube cannot function, or can function only to a limited degree. Important examinations therefore may not be able to be performed at the right time. In situations involving x-ray guided surgical interventions, life-threatening situations for the patient may arise from a sudden failure of the emitter.
In such emergencies, if the x-ray tube is of the type having two emitters (large focus and small focus), one can, in a critical case, switch over to the still-intact emitter, in order to be able to continue working even givenxe2x80x94under the circumstancesxe2x80x94greatly reduced image quality. For x-ray tubes where only one emitter is provided, this method is, of course, not possible.
A simple replacement of each x-ray tube after a specified standard life (e.g. average serviceable life) still does not solve the above problem either. This is because there are always cases in which the serviceable life of an emitter is very much shorter than the standard life, so that in such cases, the disadvantageous, sudden x-ray tube failures as described above can occur. Also, a significant safety margin from the standard life would have to be constantly maintained in order to keep the number of these cases optimally negligible for safety purposes, and a tube replacement would already have to occur in a timely fashion prior thereto, which, however, would correspondingly reduce the usage time of an x-ray tube and would raise the costs of its use accordingly.
The length of time that the serviceable life that an x-ray tube actually attains deviates from the standard life span depends considerably on the circumstances under which the x-ray tube was operated. The tube current, and thereby the electron current (emission current) emanating from the emitter, is of particular significance since x-ray tubes frequently fail due to burnout or breakage (fracture) of the emitter. At high tube currents, the temperature of the emitter, and thus also the vaporization rate at which material is vaporized from the emitter, is higher than at low tube currents. Nevertheless, it has been shown that a sufficiently exact prediction of the failure time of an x-ray tube is not possible, even if the emission current is monitored as a function of time.
This is particularly true for emitters of the type that consist of thin, e.g. only 75 xcexcm thick, sheet metal, e.g. tungsten sheet metal, since in such emitters the thermomechanical stresses which occur in the range of the emission temperature (2,350xc2x0 C. for tungsten) are already sufficient to allow the emitter to break, if it has become further thinned due to the evaporation process.
An object of the present invention is to provide an xray tube of the type initially described wherein safe use of the x-ray tube is possible until shortly before the end of the emitter life.
This object is achieved in accordance with the invention in an x-ray tube having a warning device which measures at least one electrical property of the thermionic emitter and, by analysis of the measured electrical property or properties, generates a signal, if the measured electrical property shows a value indicating an impending failure of the thermionic emitter.
The generation of the signal ensues, therefore, not on the basis of monitoring of operating parameters of the x-ray tubes; rather, it ensues on the basis of evaluation of measured electrical properties of the emitter itself, so that an exact prediction of the aging status of the emitter is possible, and thus, a use of the x-ray tube without risk is possible until shortly before the end of the emitter life.
According to a first version of the invention, the warning device measures the emitter resistance and produces a signal upon attaining a given, characteristic resistance change. This signal can serve to control a signal generator and/or be supplied to the control unit of the x-ray system, in which the x-ray tube is used, in order to institute appropriate switch-over procedures.
The change in resistance of the emitter is appropriate as a criterion for the generation of the signal because a part of its emitting substance evaporates from the surface during the aging of the thermionic emitter. The conductor cross-section thus becomes reduced, causing the emitter resistance to rise. This effect is measurable in a directly heated emitter on the basis of monitoring the filament current and/or filament voltage of the emitter. Two different possibilities to produce a signal indicating the impending emitter failurexe2x80x94via the occurrence of a resistance change since both of these parameters are dependent on a change in resistance of the emitter as a function of the operating life.
As noted above, the emitter resistance increases during the operating life. The cause is the constant vaporization of material during operation (typically 10xe2x88x928 g/(cm2xc2x7sec) for tungsten at 2,350xc2x0 C.). The conductor cross-section becomes smaller as a result and the resistance rises, which is proportionally recognizable as reduction in the filament current relative to a given filament voltage. As a first embodiment, therefore the warning device can emit a signal at a given percentage resistance increase, e.g. at a change in resistance around 10% compared to the resistance of a new emitter.
The temperature distribution of a thermionic emitter is never completely homogeneous. There are always locations that are somewhat hotter than the ambient area and more material evaporates at these hot locations. The conductor cross-section becomes more considerably reduced at such a hotshot and this ultimately leads to melting of the emitter material due to locally increased heating and thereby increased vaporization. This coupling of heating and vaporization related to melting leads to a considerably disproportional increase of the resistance in relation to the burnout life near the end of the emitter life.
Thus as a second embodiment for recognizing an impending emitter failure the warning device emits a signal when a given time gradient (rate of change) of the percentage resistance increase occurs. The repeated, considerable resistance increasexe2x80x94described abovexe2x80x94in the last operating time prior to emitter failure such as a xe2x80x9cjumpxe2x80x9d of approximately 8%, compared to the very slow resistance increase over the total life at 10%, allows the x-ray tube to be used until a few hours before the final failure of the emitter, since the considerable time gradient of the change in resistance in the last operating hours can be measured on the basis of the asymmetrical vaporization, and can be used to produce the signal indicating impending emitter failure.
In a further version of the invention, the warning device is a current measuring device, that determines the quotient of the turn-on emission current Iin when applying the tube filament voltage to the smaller equilibrium current Iequil which subsequently develops and, from this variation of the quotient during the emitter operating time, the warning device derives a signal indicating the impending failure of the emitter.
The variation of this quotient is appropriate as a criterion for the signal generation because this quotient initially changes only to a limited degree during the operating life of the tube, and increases very considerably just before the end of the serviceable life of the emitter.
If the emitter is brought to a constant emission temperature prior to switching on the high voltage, the result is the characteristic decrease of the emission current within approximately 200 ms due, to a cooling effect produced by the removal of thermal energy (corresponding to the emission temperature) due to the emitted electrons.
In the course of the serviceable life of the emitter, this becomes thinner due to evaporation as described. In this manner the thermal capacity, and the thermal conductivity due to the modified thermal conduction, decrease from the emitter interior to the emitter surface, that is considerably cooled by the electron emission which occurs after switching on the high voltage, so that the surface temperature drops accordingly and thereby the equilibrium emission current decreases. The equilibrium current is the current that would arise if the emitter were heated with the tube voltage across the emitter over a specific time. The absolute value of the equilibrium emission current depends on the emitter temperature as well as on the high voltage. Therefore, it is expedientxe2x80x94for precluding errors caused by such influencesxe2x80x94to measure not the equilibrium emission current alone, but rather to always measure the turn-on emission current Iin and the subsequently arising equilibrium currentlequil, separated by a limited time span of e.g. somewhat more than 200 ms to form the aforementioned quotient. The turn-on emission current Iin is the current that is present immediately after switching on the tube voltage after the emitter has been heated without tube voltage. Independent of the absolute values of the temperatures and voltages, this quotient Iin/Iequil is a reliable indicator for the remaining available emitter life. This quotient can be used, for example, so that a signal that indicates the impending emitter failure is emitted upon the occurrence of a predetermined percentage change of the quotient Iin/Iequil compared to the start value at the beginning of operation of the x-ray tube.
In a further embodiment of the invention, instead of using the change of the quotient Iin/Iequil as the trigger for the signal, rather the time gradient of this quotient is determined over the operating time of the emitter. It has been shown that the quotient Iin/Iequil changes considerably just before the failure of the emitter and thus a correspondingly steeper time gradient occurs. This makes it possible to generate the signal indicating the impending emitter failure in a manner that is significantly more sensitive, and coming closer to the actual end of the emitter life. The x-ray tube thus can be operated over an operating time that is almost as long as the x-ray tube life limited by the failure of the emitter, without having to take into account the disadvantages mentioned above.